The present invention relates to the study of virus-caused diseases. In particular, the invention relates to viruses that with current technology fail to multiply in cultured cells or embryonic tissues, and hence cannot be produced in quantity. Sometimes they do not produce recognizable cytopathology. Therefore their biological effects have been difficult to study. For the most part, such viruses can only be obtained from humans accidentally or voluntarily infected or from infected higher primates, only occasionally can they be obtained from infected lower species. Such viruses are termed herein non-passageable viruses, or NP-viruses, in recognition of the fact they either cannot be maintained or replicated by passage through tissue culture cells, embryonic tissues, or lower organisms or that it is difficult or impractical to do so. The diseases caused by such viruses may have long latent periods, and sometimes result in derangement of the patient's immune system or in carcinogenic transformation. Examples of such viruses include the Hepatitis B Virus (HBV), the "slow" viruses such as the causative agent of kuru, and the viral agent implicated in the etiology of multiple sclerosis, and the xenotropic viruses, such as the C-type particles implicated in the causation of certain tumors. NP viruses may be associated with chronic crippling or wasting diseases, or with cancer. In one case, HBV, there is evidence for dual pathogenicity, inasmuch as there is strong evidence linking this virus to primary carcinoma of the liver as well as to hepatitis.
In view of the serious and insidious health hazard presented by NP viruses, there is a need of a biological system of general utility to enable research on these viruses to go forward. Such a system will open an entire new research field and will provide means for the production of genetically pure viral antigens and antibodies thereto and permit production of viral components in desired amounts. The present invention provides such a biological system of general utility for enabling a vast amount of research which is currently impossible due to the nature of NP viruses. The system is also useful for the study of passageable viruses, offering the advantages of reduced biohazard, the capability to synthesize and modify specific virus-coded proteins, and to obtain quantities of viral DNA and virus-coded proteins sufficient for chemical and biochemical analysis, and for the production of vaccines. The nature of the system and the practice of the invention have been demonstrated with HBV. Further background relating to HBV, and the terminology employed in the art, will be discussed, infra.
Until recently, hepatitis has been a disease characterized primarily in terms of its symptoms and epidemiology. In 1967, Blumberg and co-workers first described an antigen associated with infection by hepatitis type B. (See, Blumberg, B. S., Science 197, 17 (1977)). Since then, extensive research has contributed a wealth of information about the disease. The causative agent is a DNA virus known as Hepatitis B Virus (HBV). The serum of infected patients contains a variety of particle types associated with infection. The whole virus particle is believed to be essentially spherical and 42 nm in diameter, comprising an envelope, a core and DNA, and termed the "Dane" particle, after its discoverer (Dane, D. S. et al., Lancet, 1970-I, 695 (1970)). The envelope contains the surface antigen (HBsAg), discovered by Blumberg. The core contains an immunologically distinct antigen, HBcAg. The DNA isolated from Dane particles is circular and contains varying length single-stranded regions, Summers, J. et al., Proc. Nat. Acad. Sci. USA 72, 4597 (1975); Landers, T. A. et al., J. Virol. 23, 368 ( 1977); Fritsch, A. et al., C. R. Acad. Sci. Paris D 287, 1453 (1978). The surface antigen is found in the serum of persons infected with HBV and in certain carrier states. Antibodies to HBsAg are found in the serum of patients who have been infected with HBV. Antibodies to the core antigen are also found in certain carrier states. A radioimmunoassay has been developed for HBsAg, Ling, C. M. et al., J. Immunol. 109, 834 (1972), and for anti-HBsAg, Hollinger, F. et al., J. Immunol. 107, 1099 (1971).
The HBsAg is an immunochemically defined material associated with the envelope of the virus. Previous studies indicate that HBsAg comprises several components of varying antigenicity, including both glycosylated and nonglycosylated proteins as major components (Peterson, D. L., et al., Proc. Nat. Acad. Sci. U.S.A. 74, 1530 (1977); Peterson, D. L., et al., in Viral Hepatitis, A Contemporary Assessment of Etiology, Epidermiology, Pathogensis and Prevention (G. N. Vyas, S. N. Cohen and R. Schmid, eds.), pp. 569-573, Franklin Institute Press, Philadelphia, 1978). In addition, lipid and several additional protein components have been reported to be present in surface antigen preparations, Shi, J. W. K. and Gerin, J. L., J. Virol. 21, 347 (1977). The major protein components were reported as having molecular weights (M.W.) of 22,000 and 28,000 daltons for the nonglycosylated and glycosylated proteins, respectively, based upon sodium dodecyl sulfate (SDS), gel electrophoresis, Peterson, et al. (1977), supra. An N-terminal sequence of 9 amino acids of the 22,000 M.W. protein, isolated from plasma of a human carrier of HBsAg by preparative SDS gel electrophoresis was reported to be Met-Glu-Asn-Ile-Thr-(Ser) or (Cys)-Gly-Phe-Leu (Peterson, et al., 1977, supra.
Standard abbreviations are used herein to denote amino acid sequences:
______________________________________ Ala = Alanine Cys = Cysteine Gly = Glycine His = Histidine Glu = Glutamic acid Lys = Lysine Gln = Glutamine Leu = Leucine Asp = Aspartic acid Ile = Isoleucine Asn = Asparagine Val = Valine Arg = Arginine .circle.M or Met = Methionine Ser = Serine Tyr = Tyrosine Thr = Threonine Phe = Phenylalanine Trp = Tryptophan Pro = Proline ______________________________________
All amino acids are in the L-configuration unless stated otherwise. In some instances herein, methionine is designated by .circle. to signify its potential role in translation initiation. An N-terminal sequence of 19 amino acids for a protein similarly isolated was reported to be: Met-Glu-Asn-Ile-Thr-Ser-Gly-Phe-Leu-Gly-Pro-Leu-Leu-Val-Ser-Gln-Ala-Gly-Ph e. (Peterson, et al., 1978, supra.) The non-glycosylated protein was reportedly immunogenic, but the glycosylated peptide, isolated as described by Peterson et al., 1977, supra, was not. However, other workers have reported a glycosylated peptide component which was immunogenic, Gerin, J. L., et al., in Viral Hepatitis, supra, pp. 147-153 (1978). The discrepancy has not been fully explained. It is known that the immunogenicity of the surface antigen proteins is sensitive to conformation changes. Possibly the use of detergents in the isolation and purification of surface antigen proteins from serum or plasma leads to diminished immunological reactivity.
The ability to detect the surface and core antigens has proven of great clinical value, especially for the screening of potential blood donors, since transfusion is one of the more common modes of HBV transmission in developed countries. Presently available sources of Dane particles for partially purified HBsAg limit the quality and quantity of antibody which can be produced. The virus cannot be grown in culture and can only be obtained from infected human patients or after infection of higher primates. Therefore, there is no means for maintaining stocks of HBV or for obtaining desired amounts of the virus or any of its components. The virus experts no cytopathic affects on cultured cells or tissues, so that no means for measurement of infective virus particles is currently available. Genetically pure HBV stocks have not been available prior to the present invention. These limitations severely restrict efforts to provide HBsAg in improved amount and quality for the production of antibody suitable for more sensitive immunoassay, for passive immunization, and antigen for active immunization. Furthermore, the inability to passage the virus outside of humans or higher primates makes it impossible to obtain sufficient antigen for the production of a vaccine. The limited host range of HBV and its failure so far to infect tissue culture cells have drastically restricted study of the virus and have hindered development of a vaccine for the serious diseases that it causes.
Recent evidence strongly indicates a link between HBV and primary hepatocellular carcinoma. Epidemiological studies have indicated a high correlation of HBsAg or HBcAg in patients with primary hepatocellular carcinoma, Trichopoulos, D. et al., Lancet, 1978, 8102. More significantly, a strain of cultured hepatocellular carcinoma cells ("Alexander" cells) is known to produce HBsAg. These cells therefore contain at least part of the HBV genome. Further elucidation of the role of HBV in hepatocellular carcinogenesis and the molecular mechanisms of the carcinogenic transformation depends upon the development of suitable biological systems for maintenance and manipulation of the virus or its genome.